Such arrangements are known from DE-PS 28 56 536, DE-OS 39 09 535, EU-A1-00 60 394, U.S. Pat. No. 4,266,673 or DE-OS 22 33 111, for example.
As follows from DE-PS 28 56 536, the objective of these arrangements is not only to separate the binder covers from the sand grains but also to separate the sand from the isolated coverings and other impurities inside the arrangement and during the treatment. In the latter citation additional outlet openings for a secondary gas stream are arranged in a certain manner about the ascending pipe for this purpose so as to obtain a local limited uniform flow-through which acts in the same direction all around. The separation means used is an impingement plate arranged at a distance over the outlet opening of the ascending pipe. Several separator cells are disposed in a joint upper housing box, an outlet chamber being arranged above the impingement plate for each separator cell, into which chambers the gas leaving the impingement plate flows together with the separated particles of material. These chambers are connected to a joint exhaust-air collector. At a distance below the outlet opening of the ascending pipe each separator has a screen-like catch means developed in such a way that it conveys a partial stream of the sand stream coming down from the impingement plate in a veil-like manner to the next separator cell while the other partial stream is supplied back again to the inlet nozzle of the ascending pipe. On the whole, this arrangement is expensive and leaves a lot to be desired during sorting as regards the separating cut.
Another drawback consists in that great forces have to be expended for separating the binder covers, owing to which the sand grains are partially destroyed at the impingement plate and the sand grain structure is changed in undesired manner.
In order to reduce the expenditure, a special swirling step is assigned to the outlet end of the ascending pipe of the arrangement according to the above-mentioned laid-open print. By this the escaping air current is to expand partially radially by taking along impurities contained in the used sand so as to impinge on the mass of sand, dust and air, which falls down from the impingement plate, thus better removing the fines. In order to improve this effect, the impingement plate extends relatively far downwardly and is constricted at its lower outlet end. The separation of the binder covers from the sand grains is not improved by this.
The object of this invention is to further develop said initially mentioned arrangement in such a way that accompanied by a space-saving design both the separation of the binder covers from the individual sand grains and sorting can markedly be improved, so that the effectiveness of the reconditioning can markedly be increased in each separator cell, the sand grain structure being maintained as completely as possible.
This problem is solved by an arrangement for the dry reconditioning of used foundry sands, in which the used sand contains binder covers and impurities and air of ambient temperature or an air/gas mixture heated to a maximum of 550.degree. C. can be supplied to a mechanical cleaning step in the form of a multi-cell separator and each separator cell having an ascending pipe open at both ends; a precipitator arranged at a distance above and opposite the upper outlet opening of the ascending pipe having an axis and comprising surfaces decelerating and deflecting the used sand/air or used sand/gas/air jet; fan means for supplying the air or the air-gas mixture; feed means for feeding the sand and conveying the sand from one separator cell to the other separator cell, respectively; means for removing the sand from the separator; means for collecting and removing the resulting covering and contaminating (impurity) substances, characterized in that the precipitator of the or each separator cell has several planar elements in addition to the surfaces decelerating and deflecting the jet rising through the ascending pipe, which elements are distributed at mutual peripheral distances about the axis of the ascending pipe; whereby each element projects into the flow of the deflected jet in a decelerating fashion thus dividing it into partial jets and swirling it.
Above all, the arrangement is suitable for the dry reconditioning in a plant in which the used sand is treated with air or an air/gas mixture of relatively low temperature reaching a maximum of 550.degree. C. preferably markedly less, so that it is possible to use above all the clay-like binders and brighteners separated from the sand grains of the used sand as binders again.
Due to the development of the novel arrangement, particularly the embodiments according to the subfeatures, the precipitator of each separator cell holds out a plurality of friction and swirling surfaces to the decelerated and deflected stream of material leaving the ascending pipe, which convey this stream of material, subdivided into a plurality of partial streams, along the friction surfaces and strongly swirl it thus causing the sand grains to get into a strongly stirring relative motion in the area of the precipitator. A high efficiency is obtained during the separation of the binder covers by the subdivision into partial streams and the strong swirling thereof in a confined space. At the same time, a classification of the solids particles is achieved in the area of the precipitator and its planar elements. The coarser particles or sand grains remain in the region of the radially planar elements while the separated binder particles and the fine impurities are conveyed and removed upwardly by the air stream flowing outwardly through the baffles formed by the planar elements. The separation of the finer particles is further improved by the subdivision into partial streams and the intense enlargement of the surface of the flowing mixture accompanied by this. The differing sand grains conveyed downwardly, while subjected to friction, along the planar elements at varying distance from the axis of the ascending pipe in a coronal fashion are exposed to a strong friction effect and further sorting when leaving the deflecting element, since part of the carrier air or carrier air/gas mixture also flows downwardly and, when leaving the planar elements of the precipitator, flows outwardly and upwardly to the outlet by conveying finer particles upwardly. This serves for obtaining not only a very intense cleaning effect but also reliable and effective sorting. Additional expansion spaces or means for supplying additional air serving for sorting are not required.
As a result, the arrangement can also be developed in a relatively simple manner and arranged in space-saving fashion.
The further subfeatures provide for an especially favorable embodiment of the precipitator. The arrangement, in the flow direction, of the cells of each separator and of the supply, discharge and transfer pipes along downwardly slanted lines, preferably in a dense fashion, leads to an especially simple control of material and a compact arrangement which can substantially be operated automatically and continuously. These advantages are further enhanced when at least part of the planar elements of the precipitator are V-shaped--viewed in the direction of sight in parallel with the axis of the ascending pipe; a sheet is arranged within the housing in the direction of a transfer pipe extending to the next separator cell, which sheet covers the housing cross-section only partially but is arranged in such a way that the used sand may slip on the sheet into the discharge transfer pipe; a shutter is disposed at the end of each transfer pipe, which is automatically closed by the sand sinking downwardly in the housing until the sand has fallen below the shutter level; and/or the precipitator element of each separator cell is arranged in a double-conical part of the separator housing in such a way that a gap having a width of at least half the radial dimension of the planar plate remains between the inner wall of this member and the periphery of the precipitator, the upper truncated cone being connected to the means for removing the separated covering substances and the like.
Further advantageous features will be apparent in the following description .